#3 Constantly being fed to Lesnar
There was no one in WWE who had managed to put The Undertaker down. When Brock Lesnar came onto the WWE scene, he was touted as being "The Next Big Thing". He went over the likes of Hulk Hogan and The Rock before kicking off a feud with Undertaker.
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Ever since then, WWE has fed their top performer to Brock Lesnar on numerous occasions. Lesnar destroyed him inside Hell in a Cell in 2002, a Biker Chain match in 2003, WrestleMania XXX, and many more. The one-time Undertaker managed to beat Lesnar was done in controversial fashion, which wasn't something that looked good on an icon like him.
Vince McMahon built The Deadman for more than a decade, only to feed him to Brock Lesnar during both of his runs in WWE. These losses sent a very clear signal to the WWE Universe: The Undertaker just isn't good enough for Brock Lesnar.